Eric Bowman - abstracto - Eric Bowman - abstracto - - 4 years ago 115
Scala Question

Best way to turn a Lists of Eithers into an Either of Lists?

I have some code like the below, where I have a list of Eithers, and I want to turn it into an Either of Lists ... in particular (in this case), if there are any Lefts in the list, then I return a Left of the list of them, otherwise I return a Right of the list of the rights.

val maybe: List[Either[String, Int]] = getMaybe
val (strings, ints) = maybe.partition(_.isLeft)
strings.map(_.left.get) match {
case Nil => Right(ints.map(_.right.get))
case stringList => Left(stringList)
}


Calling
get
always makes me feel like I must be missing something.

Is there a more idiomatic way to do this?

Answer Source
data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings) yield s)
}

//For one pass:

data.partition(_.isLeft) match {                            
  case (Nil,  ints) => Right(for(Right(i) <- ints.view) yield i)        
  case (strings, _) => Left(for(Left(s) <- strings.view) yield s)
}
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